1. Field of the Invention
The present invention generally relates to a switching device, and more particularly to a current-induced magnetic switching device for use with a nonvolatile memory array that uses magnetic memory elements as the individual memory cells.
2. Description of the Related Art
Magnetic random access memory (MRAM or typically referred to as "MagRam") technology is a solid state device technology using magnetic thin film elements as a storage mechanism. The storage mechanism relies on the relative orientation of the magnetization of two electrodes, and on the ability to discern this orientation by electrical means.
MRAM arrays include an array of magnetic memory cells positioned at the intersections of wordlines and bitlines. Generally, each cell includes a magnetically changeable or "free" region, and a proximate magnetically reference region, arranged into a magnetic tunnel junction ("MTJ") device (e.g., the term "reference region" is used broadly herein to denote any type of region which, in cooperation with the free or changeable region, results in a detectable state of the device as a whole).
Generally, the principle underlying storage of data in such cells is the ability to change the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis ("EA") of the free region, and the ability to thereafter read this relative orientation difference.
More particularly, an MRAM cell is written by reversing the free region magnetization using applied bi-directional electrical and resultant magnetic stimuli via its respective bitline and wordline.
The MRAM cell is later read by measuring the resultant tunneling resistance between the bitline and wordline, which assumes one of two values depending on the relative orientation of the magnetization of the free region with respect to the reference region. If the free region is modeled as a simple elemental magnet having a direction of magnetization which is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or -EA), and if the reference region is a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states (and therefore the two possible tunneling resistance values) are defined for the cell: aligned (+EA/+EA) and anti-aligned (-EA/+EA).
Thus, in operation as a memory device, the MRAM device can be read by measuring the tunneling resistance, thereby to infer the magnetization state of the storage layer with respect to the fixed layer. The MRAM can be written by reversing free layer magnetization using external magnetic fields. If the free layer is imagined as a simple elemental magnet which is free to rotate but with a strong energetic preference for aligning parallel to the X axis, and if the pinned layer is a similar elemental magnet but frozen in the +X direction, then there are at least two states possible for the device (e.g., aligned and anti-aligned) (i.e., in +X or -X directions).
Thus, a magnetic random access memory (RAM) requires write operations on small ferromagnetic elements. The conventional way of write-addressing has been to use an x-y cross-current excitation, which requires large write current, demands stringent magnetic switching characteristic from the memory element, and has cross-talk problems upon the increase of memory density.
Further, the conventional structures and methods do not allow for high packing density without cross-talk. Further, the driving circuits become complex due to the x-y selective magnetic-field induced write operation. Thus, conventional magnetic random access memory (RAM) requires read-write operations on small ferromagnetic elements, and have any problems.
A mechanism for the direct switching of the magnetic memory element has been proposed in J. C. Slonczewski, J. Magn. and Magn. Mat. 159, LI (1996), which is based on the theoretical prediction of a new effect due to interactions between spin-polarized conduction electrons and the ferromagnetic moments of the electrodes. However, such a proposal was strictly theoretical.
Further, in M. Tsoi et al., Phys. Rev. Lett., 80, 4281 (1998), a point-contact device was constructed to show an anomaly in its current-voltage characteristic, which could be interpreted as spin-wave excitation due to the momentum transfer effect mentioned above. However, such a model was strictly hypothetical.
Further, it is noted that recently in manganite trilayer junctions, large low-field magnetoresistance (MR), of up to an order of magnitude change in resistance, was observed at 14.degree. K. in 100 Oe. The junctions are made of epitaxial La.sub.0.67 (Sr/Ca).sub.0.33 MnO.sub.3 (LSMO or LCMO) thin film electrodes with a SrTiO.sub.3 (STO) barrier. These manganites are expected to be half-metals when their ferromagnetic order is fully developed. Band calculations show that their minority band has a very low carrier concentration, making it prone to disorder-induced localization.
According to a spin-dependent tunneling model, a half-metallic metal-insulator-metal junction would exhibit large, almost infinite MR. However, the observed transport characteristics do not resemble a clean metal-insulator-metal toweling junction. The resistance varies strongly with temperature, especially above 130.degree. K. The MR decreases rapidly as temperature increases, vanishing above 130.degree. K., well below the Curie temperature of the thin film electrodes which is around 360.degree. K. The MR is bias-dependent, suppressed by a voltage of around 0.2V. Inhomogeneities of transport current at the junction interface is suspected, and the exact mechanism for such large low-field MR is not well understood.